function animate(obj,target,callback){
        clearInterval(obj.timer);
        obj.timer= setInterval(function(){
            var step=(target-obj.offsetLeft)/10;
            step= step < 0? Math.floor(step) : Math.ceil(step);
            if(obj.offsetLeft==target){
                clearInterval(obj.timer);
                if(callback){
                    callback();
                }
            }
            obj.style.left= obj.offsetLeft+ step +'px';
        }, 15);
}